Wafer carrier mapping sensors are used in the manufacture and processing of semiconductor wafers to detect wafers in a wafer carrier. In order to effectively process the wafers in the carrier, the semiconductor tool must know which positions within the carrier have wafers, and whether those wafers are properly held therein. Wafer carrier mapping sensors scan the wafer edges to detect wafer absence/presence and additionally errors such as cross slots. “Cross slots” are wafer position errors where the wafer is not maintained in a single “slot” but erroneously spans a pair of slots. In this situation, it is important that the tool not attempt to interact with the wafer, but instead generate an error.
Wafer carrier mapping systems have begun to face a difficult challenge recently with the increasing use of dark wafers typically with nitride coatings. While mapping systems can be adjusted to optimally detect the traditionally light wafers, such as copper or aluminum coated silicon wafers, or the dark nitride wafers, sensing systems do not adequately deal with mixes of both light and dark wafers. For example, if a system is optimally adjusted for dark wafers, the illumination level and detector gain may be so high that when the sensing system is faced with a light wafer, an erroneous signal such as a false cross slot error is declared. Conversely, when a system is optimally adjusted for light wafers, a dark wafer may fail to register well enough to make its presence known. Unfortunately, adjusting the sensing system to compromise between these two extremes essentially creates a worse situation in which false cross slot errors still occur, and some dark wafers are not detected at all.
There exists a need to provide a wafer carrier mapping system that can reliably detect not only wafer presence within a carrier containing a mixture of light and dark wafers but also reduce or eliminate false errors. Such a system would reduce technician intervention since one sensing system would accommodate both types of wafers. Moreover, sensor credibility would be increased since technicians would not need to second guess each cross slot error to determine if it was in fact a false error.